From perovskite top interfaces to metal contacts: stability-driven design for tandem-compatible inverted solar cells

Inverted perovskite solar cells (PeSCs) have attracted growing attention, due to their low-temperature processability, reduced hysteresis, and strong compatibility with flexible and tandem device architectures, and have now surpassed conventional devices in terms of both power conversion efficiency and operational stability. Despite these advantages, their long-term operational stability remains critically constrained by the upper device stack, spanning from the perovskite surface through the electron-transporting layer (ETL) to the metal cathode. This review systematically analyzes the dominant origins of instability at these top interfaces in inverted PeSCs and organizes recent progress into three key design directions: (i) perovskite/ETL interface engineering using organic and inorganic modifiers, low-dimensional perovskite capping layers, and thermally conductive interlayers; (ii) C₆₀ replacements or non-fullerene ETLs that integrate defect passivation, interfacial dipole tuning, ion management, and aggregation-resistant morphologies; and (iii) ETL/cathode interlayers that regulate band bending, suppress interfacial recombination, and block volatile halide migration. Finally, the Outlook section discusses emerging opportunities and remaining challenges in further optimizing ETLs and top interlayers, with particular emphasis on advancing the operational stability and commercial viability of inverted PeSCs and their integration into high-performance tandem solar cells.